Question: The lifespans of gorillas in a particular zoo are normally distributed. The average gorilla lives $20.8$ years; the standard deviation is $3.1$ years. Use the empirical rule (68-95-99.7%) to estimate the probability of a gorilla living less than $23.9$ years.
Answer: $20.8$ $17.7$ $23.9$ $14.6$ $27$ $11.5$ $30.1$ $68\%$ $16\%$ $16\%$ We know the lifespans are normally distributed with an average lifespan of $20.8$ years. We know the standard deviation is $3.1$ years, so one standard deviation below the mean is $17.7$ years and one standard deviation above the mean is $23.9$ years. Two standard deviations below the mean is $14.6$ years and two standard deviations above the mean is $27$ years. Three standard deviations below the mean is $11.5$ years and three standard deviations above the mean is $30.1$ years. We are interested in the probability of a gorilla living less than $23.9$ years. The empirical rule (or the 68-95-99.7 rule) tells us that $68\%$ of the gorillas will have lifespans within 1 standard deviation of the average lifespan. The remaining $32\%$ of the gorillas will have lifespans that fall outside the shaded area. Because the normal distribution is symmetrical, half $({16\%})$ will live less than $17.7$ years and the other half $({16\%})$ will live longer than $23.9$ years. The probability of a particular gorilla living less than $23.9$ years is ${68\%} + {16\%}$, or $84\%$.